Quantum dots which are ultrafine particles of 10 nm or less in particle size have an excellent carrier (electrons, holes) confinement capability, and thus, can easily generate excitons through recombination of electron-hole. For this reason, luminescence from free excitons can be expected, and it is possible to achieve luminescence with a high luminance efficiency and a sharp luminescence spectrum. In addition, quantum dots can be controlled in a wide range of wavelengths through the use of quantum size effect, and thus have been attracting attention for application to light emitting devices such as semiconductor lasers and light emitting diodes (LED).
Incidentally, colloidal quantum dots are chemically synthesized in a liquid phase, and typically have surfaces covered with organic molecules of a surfactant so as to prevent agglomeration of the quantum dots with each other. Therefore, the colloidal quantum dots have a high electric potential barrier because of the low conductivity of the surfactant due to the organic molecules, and for this reason, have the drawback of being low in photoelectric conversion efficiency through carriers (hole and electrons). In addition, when a conductive polymer or a metallic material is used as the surfactant, the carriers injected into electrodes by voltage application will pass through the surfactant from the positive electrode to the negative electrode or from the negative electrode to the positive electrode, and it is thus difficult to efficiently confine the carriers in the quantum dots.
FIG. 13 is a schematic diagram of a photoelectric conversion device on the assumption of the use of a conductive surfactant.
This photoelectric conversion device have a quantum dot layer 105 interposed between a hole transport layer 102 formed on the upper surface of a positive electrode 101 and an electron transport layer 104 formed on the lower surface of a negative electrode 103. Further, this quantum dot layer 105 has a surface coated with a conductive surfactant 109, so as to prevent agglomeration of quantum dots 108 with each other, which are each composed of a core section 106 and a shell section 107. More specifically, the quantum dot layer 105 has a stacked structure with a large number of quantum dots 108 provided in lines, and the surfactant 109 is interposed between the quantum dots 108.
Then, when a voltage is applied between the positive electrode 101 and the negative electrode 103, holes are injected into the positive electrode 101, whereas electrons are injected into the negative electrode 103. Then, the holes and electrons as carriers passes through the conductive surfactant 109, and without being confined in the quantum dots 108, the holes are transported in a direction to the negative electrode 103, whereas the electrons are transported in a direction to the positive electrode 101, as indicated by an arrow a and an arrow b. More specifically, when the conductive surfactant 109 is used, the carriers will merely provide conduction, and it is not possible to confine the carriers in the quantum dots 108.
Furthermore, techniques have been also researched and developed, which are adapted to use a surfactant with both hole transporting and electron transporting ligands.
For example, Patent Document 1 proposes a nanoparticle luminescent material including a surfactant composed of at least two types of ligands localized on the surfaces of quantum dots, wherein at least one of the ligands is a hole transporting ligand, whereas at least one thereof is an electron transporting ligand.
In Patent Document 1, the coordination of both the electron transporting ligand and hole transporting ligand on the surfaces of the nanoparticles allows the suppression of charge transport between the ligands, thereby improving the efficiency of charge injection into the nanoparticles.
In addition, in Patent Document 1, a dispersed solution of quantum dots as nanoparticles is prepared by a method as shown in FIG. 14.
First, in a raw material solution preparation step 111, a chloroform dispersed solution of CdSe nanoparticles is prepared. Specifically, a toluene dispersed solution of CdSe nanoparticles which have surfaces coated with TOPO (trioctylphosphineoxide) is stirred with the addition of methanol, and then subjected to centrifugation to produce CdSe nanoparticles, and after removing the supernatant solution, the precipitated CdSe nanoparticles are subjected to drying, and then the addition of chloroform to prepare a chloroform dispersed solution of the CdSe nanoparticles, that is, a raw material solution.
Then, in a surfactant addition step 112, a surfactant containing a hole transporting ligand (for example, an α-NPD derivative) and a surfactant containing an electron transporting ligand (for example, BPhen) are added to the raw material solution.
Then, in a ligand replacement step 113, stirring is carried out in a nitrogen atmosphere for a predetermined period of time under the conditions of room temperature and light shielding, and then the resultant is allowed to stand still to conduct a ligand replacement operation, followed by coating the surfaces of the CdSe nanoparticles with the hole transporting surfactant and the electron transporting surfactant.
Subsequently, in a suspended ligand removal step 114, unwanted ligands are removed which are replaced and suspended in the solution. This suspended ligand removal step 114 has two treatment steps of a poor solvent addition treatment 114a and a supernatant solution removal treatment 114b, in which an appropriate amount of poor solvent such as methanol is added to produce a precipitate in the poor solvent addition treatment 114a, and in the subsequent supernatant solution removal treatment 114b, the suspended ligands are removed along with the supernatant solution. Then, a series of treatment steps composed of the poor solvent addition treatment 114a and the supernatant solution removal treatment 114b is repeated more than once, thereby purifying a powder of CdSe fine particles.
Then, in a redispersion step 115, a dispersion solvent such as chloroform is added to the powder of CdSe fine particles for the redispersion of the powder, thereby providing a transparent quantum dot dispersed solution in which the nanoparticle luminescent material is dispersed.
More specifically, when the ligand replacement is carried out in the solution in which the hole transporting ligand and the electron transporting ligand coexist, replaced unwanted ligands are suspended in the solution. Therefore, when this solution is used directly to prepare a thin film, there is a possibility that a large number of suspended ligands will penetrate the film to impair the function.
Thus, in Patent Document 1, a series of treatment step of: adding the poor solvent to produce a precipitate in the poor solvent addition treatment 114a of the suspended ligand removal step 114; and removing the supernatant solution to remove unwanted suspended ligands in the subsequent supernatant solution removal treatment 114b, is repeated more than once to completely remove the suspended ligands, and the powder is then dispersed in the dispersion solvent to obtain the quantum dot dispersed solution.
Patent Document 1: Japanese Patent Application Laid-Open No. 2008-214363 (claim 1, paragraphs [0078] and [0079])